The present invention relates to nonlinear resistive devices, such as varistors, and more particularly to methods of making such devices using controllable plating techniques in which the exposed end terminals of the device are plated with nickel barrier terminations while the exposed semiconductor body remains unplated.
Nonlinear resistive devices are known in the art, and are described, for example, in U.S. Pat. No. 5,115,221 issued to Cowman on May 19, 1992, and is incorporated by reference.
With reference to the prior art shown in FIG. 1, a typical device 10 may include plural layers 12 of semiconductor material with electrically conductive electrodes 14 between adjacent layers. A portion of each electrode 14 is exposed in a terminal region 16 so that electrical contact may be made therewith. The electrodes 14 may be exposed at one or both of opposing terminal regions, and typically the electrodes are exposed at alternating terminal regions 16 as illustrated. The exposed portions of the electrodes 14 are contacted by electrically conductive end terminals 18 that cover the terminal regions 16.
While an apparently simple structure, the manufacture of such devices has proved complex. For example, the attachment of the end terminals 18 has proved to be a difficult problem in search of a simplified solution. Desirably, the terminal regions 16 may be plated with nickel and tin-lead metals to increase solderability and decrease solder leaching. The process parameters in plating nickel to zinc oxide semiconductor bodies has proved particularly vexing and has required complex solutions.
One method of affixing the end terminals 18 is to use a conventional barrel plating method in which the entire device is immersed in a plating solution. However, the stacked layers are semiconductor material, such as zinc oxide, that may be conductive during the plating process so that the plating adheres to the entire surface of the device. Thus, in order to provide separate end terminals as shown in FIG. 1, a portion of the plating must be mechanically removed after immersion, or covered before immersion with a temporary plating resist comprised of an organic substance insoluble to the plating solution. However, the removal of the plating or organic plating resist is an extra step in the manufacturing process, and may involve the use of toxic materials that further complicate the manufacturing process.
It has also been suggested that the metal forming the end terminals 18 be flame sprayed onto the device, with the other portions of the surface of the device being masked. Flame spraying is not suitable for many manufacturing processes because it is slow and includes the creation of a special mask, with the additional steps attendant therewith. See, for example, U.S. Pat. No. 4,316,171 issued to Miyabayashi, et al. on Feb. 16, 1982.
It is also known to react a semiconductor body, having electrically conductive metal end terminations, with phosphoric acid to selectively form a phosphate on the semiconductor body prior to providing end terminations using conventional barrel plating. See, U.S. Pat. No. 5,614,074 issued to Ravindranathan on Mar. 25, 1997.
As illustrated by the above known methods, a simplified manufacturing process for the attachment of the end terminals 18 has proved to be a illusive.
Accordingly, it is an object of the present invention to provide a novel method and device that obviates many of the prior art problems.
Accordingly, it is an object of the present invention to provide a novel method of manufacturing a semiconductor device by controllably reacting an exposed zinc oxide semiconductor device having an exposed end terminal region with a nickel plating solution to form a nickel barrier end termination over the semiconductor body end without plating the entire exposed semiconductor device.
It is another object of the present invention to provide a novel method of providing a semiconductor device by controllably partially immersing an exposed semiconductor body having a silver termination with a nickel plating solution while applying a biasing current to form a nickel barrier cap extending a selected distance up the exposed body of the semiconductor device.
It is yet another object of the present invention to provide a novel method of providing a semiconductor body with a nickel barrier cap without the use of a plating resist by positioning an exposed end of the semiconductor body a selectable distance into a nickel plating solution for a controlled period.
It is still another object of the present invention to provide a novel method of providing metal termination of an exposed semiconductor body by contacting an end of the semiconductor body with an absorbent material impregnated with a nickel plating solution.
It is a further object of the present invention to provide a novel semiconductor device having naturally formed nickel terminations over a body of resistive plates interleaved between zinc oxide layers.
It is yet a further object of the present invention to provide a novel method of directly nickel plating zinc oxide bodies having a preferred zinc oxide volume resistivity for the plating method selected.
It is still a further object of the present invention to provide a novel method of manufacturing zinc oxide semiconductor devices minimizing solder leaching by providing a platinum-free and palladium-free silver termination and thereupon forming a nickel barrier termination.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.